Although food could be obtained at any time during a trial, the response more likely to produce it depended on how much time had elapsed since the start of that trial. At the beginning of each trial, one response (a peck to the left or right key, depending on the condition) was nine times more likely to produce food than the other response. Once a fixed amount of time had elapsed since the start of a trial, the probability of producing food reversed between the two responses. Thus, one key had a higher probability of delivering food before a reversal, and a lower probability of delivering a reinforcer after a reversal – we term this the Higher-to-Lower (H → L) key. The other, Lower-to-Higher (L → H) key had a lower probability of delivering food before a reversal, and a higher probability after a reversal.